Signal actuated alarm circuit



June 30, 1942. ZEPLER 2,287,926

SIGNAL ACTUATED ALARM CIRCUIT Filed Feb. 23, 1959 I INVENTOR ERICH ERNST ZEPLER ATTORNEY Patented June 30, 1 942 SIGNAL ACTUATED ALARM CIRCUIT Erich Ernst Zepler, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application February 23, 1939, Serial No. 257,901 In Great Britain March 4, 1938 Claims.

This invention relates to signal actuated alarm call and like arrangements and more particularly to such arrangements of the kind wherein actuation is automatically obtained upon the reception of a continuous signal exceeding a predetermined length.

It is well known to operate an alarm or call device circuit by means of a receiver arranged to complete the said circuit upon reception of a continuous signal exceeding a predetermined length, e. g. a dash exceeding 30 seconds in length. The usual practice is to utilize the received signal to charge a condenser through a resistance, the said condenser and resistance being dimensioned to provide a predetermined time constant, and the call or alarm device circuit being actuated when the condenser reaches a predetermined state of charge. An important practical defect of known arrangements of this type is that the alarm or call circuit may be improperly operated as a result of the reception of interfering Morse signals, especially if the Morse speed is high so that the time intervals between successive signals is small.

The principal object of this invention is to avoid this defect.

According to this invention a signal actuated call or alarm arrangement of the kind wherein actuation of the call or alarm is required to be effected upon reception of a continuous signal exceeding a predetermined minimum time and wherein said actuation is secured in dependence upon the state of charge of a condenser which is arranged to be charged at a predetermined rate under the control of received signals is charac terized by the provision of means for automatically restoring said condenser to a state of charge at or near the datum state upon occurrence of a gap or break in a received signal.

The invention is illustrated in the accompanying drawing which shows diagrammatically one embodiment thereof.

Referring to the drawing, a call or alarm circuit (not shown) is controlled by the contacts I of a relay whose winding 2 is included in the anode circuit of a thermionic valve 3. A main condenser 4 .is connected directly between the grid 5 and cathode 6 of valve 3 and the said grid 5 is also connected to the said cathode 6 through a series circuit consisting, in the order stated, of a first rectifier l with a shunt resistance 8 of high value connected across it, a second resistance 9 and a source ID of potential, the said source having its negative terminal towards the pass direction such that it presents high resistance for currents flowing towards the grid 5. Received signals obtained in any convenient way, are applied to the primary ll of a transformer whose secondary I2 is connected at one end directly to the junction of the second resistance 9 withthe source In of potential, the other end of said secondary being connected through a second rectifier I3 to the other end of said second resistance. The second rectifier is connected with its pass direction such that it presents low resistance to incoming signals tending to make the grid end of the second resistance 9 positive. A second condenser I4 is connected directly across the second resistance 9. The valve is so operated that in the absence of a predetermined positive voltage across the main condenser 4 no anode current flows and the relay l--2 is accordingly not operated. When a signal is being received the signal amplitude produces across the second resistance 9 a positive voltage which results in a change of potential across the main condenser 4 this change of potential taking place at a rate depending upon the time constant set (substantially) by the value of the main condenser 4 and the sum of the values of the shunt resistance 8 and the second resistance 9. This time constant may be chosen to be of the order of, say, 10 to 30 seconds (depending upon requirements). The time constant determined by the second condenser l4 and the second resistance 9 in conjunction is very small. Thus, if a received signal continues unbroken long enough, the main condenser 4 will become charged to a potential such that anode current flows in the valve 3 and the relay l2 is operated. If, however, there is even momentary interruption of the signal, the positive voltage across the second resistance 9 is removed and the main condenser 4 accordingly changes its state of charge rapidly to or near the datum condition, this rate of change being determined by the time constant set by the value of the said main condenser 4 in conjunction with the resistance of the first rectifier 1 operating in the pass direction. This time constant is, of course, very small due to the low resistance offered by the said first rectifier 7 when operating in the pass direction.

Experiment with a circuit as above described has shown that faulty operation is not obtained even if long dashes with short intervals are received.

Having now particularly described and ascergrid 5. The rectifier I is connected with its tained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:

1. In combination, a thermionic discharge tube having grid and anode circuits connected thereto, a normally charged condenser in said grid circuit, a first and second resistor connected in series across said condenser, means for normally biasing said tube to cut-off, signal input means, means for rectifying said signal, means for applying said rectified signal across the first of said resistors whereby said condenser is slowly discharged through said second resistor and a unidirectiona1 conductance across said second resistor, said unidirectional conductance being so poled that upon cessation of said signal the charge on said condenser is quickly restored to normal, and the cut-off bias is restored to thegrid of said tube.

2. In combination. a thermionic discharge tube having grid and anode circuits connected thereto, means in said grid circuit for normally biasing said tube to cut-off, a normally charged condenser in said grid circuit, means responsive to a rectified signal impulse for gradually discharging said condenser, thereby overcoming said normal bias, said means comprising a first and a second resistor connected in serie in said grid circuit, and means including a unidirectional conductance connected across said first resistor for hastening the restoration of a normal charge on said condenser upon termination of said rectified signal impulse.

3. In combination, a thermionic discharge tube having grid and anode circuits connected thereto,

a local relay connected in the anode circuit,

across said second resistor for hastening the restoration of a normal charge on said condenser upon cessation of applied signal input energy.

4. A relay system comprising an electron discharge tube having grid and anode circuits connected to its cathode, the anode circuit including a responsive device and the grid circuit including two series-connected resistors in series with means for normally biasing said tube to cut-off, a rectifier in shunt with that one of said resistors which leads directly to the grid, a capacitor directly connected between the cathode and grid, and means responsive to signals for quickly applying a positive impulse to said grid circuit at the junction between said resistors, which impulse is rendered slowly effective in overcoming the cut-off bias on said tube, due to the time constant value of said capacitor and said resistors, said rectifier constituting means quickly effective to restore said cut-off bias together with the normal charge on said capacitor upon the termination of said impulse.

5. In combination, an electron discharge tube having a cathode-grid circuit and a cathodeanode circuit, and a relay Winding included in said cathode-anode circuit, means included in said cathode-grid circuit for normally biasing said tube to cut-ofi, a signal input circuit having two impedances series connected between said biasing means and the grid of said tube, a unidirectional conductance in shunt with that one of the impedances which connects directly to the grid, means for impressing a rectifier signal intermittently across the other of said impedances, thereby to oppose said cut-off biasing means, means including a capacitor directly connected between the cathode and grid of said tube for restoring a component of said rectified signal during a predetermined period in order to delay the actuation of said relay, and means for causing the stored component to be quickly dissipated through said undirectional conductance upon cessation of the signal.

ERICI-I ERNST ZEPLER. 

